Leukemic guinea pigs put into remission with 125 mg/kg of cytoxan followed by the administration of admixture of C. parvum (.5 mg) plus x-irradiated blast cells (5 times 10 to the 7th power) weekly for 2 weeks and then given vaccine alone at 3 weeks resulted in a 100% increase in MST when compared to animals that received drug alone (MST equals 49 days). A similar approach utilizing BCG (2 times 10 to the 7th power organisms) in the place of C. parvum resulted in a greater extension in MST when compared to the drug treated group but less efficacious than C. parvum. A soluble antigen extracted from the surface of leukemic blast cells at a dose of 1 mg of protein completely protected animals from a viable syngenic tumor challenge of 2 times 10 to the 5th power cells where doses of .5 and .25 mg's of the antigen afforded 83 and 66% protection, respectively. Interestingly, in vivo and in vitro immunological investigations revealed specific recognition to the soluble antigen preparation.